1. Field of the Invention
The present invention relates generally to the field of thermal imaging. More particularly, the implementation of the present invention relates to systems that include a scanning antenna for thermal imaging. Specifically, a preferred implementation of the present invention relates to a system that includes a rotatable millimeter wavelength (MMW) radial scanning antenna for fire detection within a perimeter area. The present invention thus relates to fire detection systems of the type that can be termed scanning thermal imaging.
2. Discussion of the Related Art
Historically, interior fire sensors have been based on smoke detectors. In some instances, these smoke detectors have been supplemented with an infrared (IR) heat detector in a combined fire sensor system. However, both smoke detectors and infrared heat detectors are not readily applicable to outdoor fire detection, which requires remote sensing over relatively large areas.
In the past, various kinds of optical sensors have also been used for the purpose of interior fire detection. The two wavebands generally used by such optical sensors are the infrared waveband and the ultraviolet (UV) waveband. Both of these wavebands have advantages and disadvantages. Systems that operate in the ultraviolet band have fast response times, but are subject to false alarms. Systems that operate in the infrared band have fewer false alarms, but have slow response times. Often these two bands are used together in one combined fire detection system so that the negative aspects of one band are compensated for by the positive aspects of the other band.
More recently, commercial optical sensors for fire detection have used multiple infrared wavelengths. Referring to FIG. 1, the typical emission spectrum from a typical hydrocarbon fire is high in infrared content. Using multiple infrared bands (e.g., 2.7 .mu.m and 4.3 .mu.m) improves detectability and reduces false alarms. The 4.3 .mu.m infrared band takes advantage of the distinctive CO.sub.2 emissions created by most fires. In addition, a shorter wavelength is often also selected (e.g., 0.2 .mu.m). This shorter wavelength is generally used to look for "flicker" (i.e., fast nonperiodic signals) and to provide a basis for comparison with the infrared bands. In operation, signal strength ratios are taken between the bands and, when predetermined criteria are met, an alarm is actuated. Flicker is an important discriminant because fire is one of the very few blackbody radiators that exhibits flicker. However, even in combination, the infrared and ultraviolet bands do not provide the capability of detecting a fire outdoors, especially under adverse atmospheric conditions such as fog, rain, or snow. The prior art optical fire sensors are short-range, wide-field-of-view devices (e.g., a range of from 16 meters to 25 meters and a field of view of approximately +/- 90 degrees). Even the best, prior art optical fire sensors are limited to a range of less than 70 meters.
As is known to those of skill in the art, the detection of fire outdoors requires different spectral band criteria than detection indoors, and the right combination of signal bands must be selected. For example, the solar radiation reaching the earth's surface creates a high background level across a large portion of the spectrum.
As is also known to those of skill in the art, optical fire sensors are essentially nonimaging and, therefore, do not provide any information about the location of a fire, even after the presence of a fire is detected. Needless to say, the location of a growing fire is crucial information in advanced fire detection where fire fighting resources need to be directed to the location of the fire while it is still small. Nonimaging sensors can only signal a zone alarm which carries no information as to the exact position of a fire.
Moreover, the smoke and other particles generated by a fire severely hamper any optical fire detection technique that relies on emissions in the infrared or visible or ultraviolet spectral bands. Smoke and airborne particulate matter both absorb and scatter signals in these spectral bands. Further, the presence of fog, rain or snow can completely absorb signals from fires on the shorter, optical wavelengths.